1. Edward J. Tarbuck & Frederick K. Lutgens
Earth Science, 10e
Edward J. Tarbuck & Frederick K. Lutgens

2. Plate Tectonics Chapter 7
Earth Science, 10e
Stan Hatfield and Ken Pinzke
Southwestern Illinois College

3. Continental drift: an idea before its time
Alfred Wegener
First proposed hypothesis, 1915
Published The Origin of Continents and Oceans
Continental drift hypothesis
Supercontinent called Pangaea began breaking apart about 200 million years ago
Continents "drifted" to present positions
Continents "broke" through the ocean crust

4. Pangaea approximately 200 million years ago

5. Continental drift: an idea before its time
Wegener's continental drift hypothesis
Evidence used by Wegener
Fit of South America and Africa
Fossils match across the seas
Rock types and structures match
Ancient climates
Main objection to Wegener's proposal was its inability to provide a mechanism

6. Wegener’s matching of mountain ranges on different continents

7. Paleoclimatic evidence for Continental Drift

8. Plate tectonics: the new paradigm
More encompassing than continental drift
Associated with Earth's rigid outer shell
Called the lithosphere
Consists of several plates
Plates are moving slowly
Largest plate is the Pacific plate
Plates are mostly beneath the ocean

9. Plate tectonics: the new paradigm
Asthenosphere
Exists beneath the lithosphere
Hotter and weaker than lithosphere
Allows for motion of lithosphere
Plate boundaries
All major interactions among plates occur along their boundaries

10. Convergent: Continental to Continental
Convergent plate boundaries (destructive margins)
Definition: When subducting plates contain continental material, two continents collide
How they move: two continental plates push each other up
Landform Results: new mountain ranges
Example: Himalayas
Density: Same density so they both push each other up

11. A continental-continental convergent plate boundary

12. The collision of India and Asia produced the Himalayas (before)

13. The collision of India and Asia produced the Himalayas (after)

14. Convergent Plate Boundaries: Oceanic to Continental
Convergent plate boundaries (destructive margins)
Oceanic-continental convergence
Definition: an oceanic plate and continental plate colliding
How they move: oceanic plate slides underneath continental plate Denser oceanic slab sinks into the asthenosphere
Land formations: Pockets of magma develop and rise, continental volcanic arcs form, chain of volcanic islands
Examples include the Andes, Cascades, and the Sierra Nevadan system

15. An oceanic-continental convergent plate boundary

16. Convergent Plate Boundaries: Oceanic to Oceanic
Oceanic-oceanic convergence
Definition: Two oceanic slabs converge and one descends beneath the other
How they move: Often forms volcanoes on the ocean floor
Land formation results: Volcanic island arcs forms as volcanoes emerge from the sea
Examples: include the Aleutian, Mariana, and Tonga islands,
Density: same

17. An oceanic-oceanic convergent plate boundary

18. Divergent Plate Boundaries
Divergent plate boundaries (constructive margins)
Definition: Two plates move apart
How they move: Mantle material upwells to create new seafloor
Landform Results: Ocean ridges and seafloor spreading
Oceanic ridges develop along well-developed boundaries
Along ridges, seafloor spreading creates new seafloor
Examples: Iceland, and East Africa
Density: Same density

19. Divergent boundaries are located mainly along oceanic ridges

20. The East African rift – a divergent boundary on land

21. Transform Boundaries
Definition: Plates slide past one another, horizontally (side to side)
No new crust is created
No crust is destroyed
How they move: At the time of formation, they roughly parallel the direction of plate movement
Aid the movement of oceanic crustal material
Examples: San Andres fault in California
Density: Plates have the same density

22. Plate tectonics: the new paradigm
Evidence for the plate tectonics model
Paleomagnetism
Probably the most persuasive evidence
Ancient magnetism preserved in rocks
Paleomagnetic records show
Polar wandering (evidence that continents moved)
Earth's magnetic field reversals
Recorded in rocks as they form at oceanic ridges

23. Apparent polar-wandering paths for Eurasia and North America

24. Paleomagnetic reversals recorded by basalt flows at mid-ocean ridges

25. Plate tectonics: the new paradigm
Evidence for the plate tectonics model
Earthquake patterns
Associated with plate boundaries
Deep-focus earthquakes along trenches provide a method for tracking the plate's descent
Ocean drilling
Deep Sea Drilling Project (ship: Glomar Challenger)

26. Distribution of earthquake foci at plate boundaries

27. Earthquake foci in the vicinity of the Japan trench

28. Plate tectonics: the new paradigm
Evidence for the plate tectonics model
Hot spots
Rising plumes of mantle material
Volcanoes can form over them
e.g., Hawaiian Island chain
Chains of volcanoes mark plate movement

29. Plate tectonics: the new paradigm
Evidence for the plate tectonics model
Ocean drilling
Age of deepest sediments
Youngest are near the ridges
Older are at a distance from the ridge
Ocean basins are geologically young

30. The Hawaiian Islands have formed over a stationary hot spot

31. Plate tectonics: the new paradigm
Measuring plate motion
By using hot spot “tracks” like those of the Hawaiian Island - Emperor Seamount chain
Using space-age technology to directly measure the relative motion of plates
Very Long Baseline Interferometry (VLBI)
Global Positioning System (GPS)

32. Directions and rates of plate motions

33. Plate tectonics: the new paradigm
Driving mechanism of plate tectonics
No one model explains all facets of plate tectonics
Earth's heat is the driving force
Several models have been proposed
Slab-pull and slab-push model
Descending oceanic crust pulls the plate
Elevated ridge system pushes the plate

34. Plate tectonics: the new paradigm
Driving mechanism of plate tectonics
Several models have been proposed
Plate-mantle convection
Mantle plumes extend from mantle-core boundary and cause convection within the mantle
Models
Layering at 660 kilometers
Whole-mantle convection
Deep-layer model

35. Layering at 660 kilometers

36. Whole-mantle convection

37. Deep-layer model

38. End of Chapter 7